1. Field of the Invention
The present invention relates to the hemming arts. More particularly, the present invention relates to improvements in a hemming apparatus and an improved method for hemming sheet metal or like material together. The present invention finds particular application in the automotive field and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
2. Discussion of the Art
In the automotive field it is often desirable to join two pieces of sheet metal together in a hemming operation to form a door, hood, trunk deck, or other such component. Generally, a unitary outer skin of sheet metal is hemmed to a second inner reinforcing panel of sheet metal. Hemming involves bending and compressing a generally upturned or perpendicular flange located along each edge of the outer sheet over an adjacent edge of the inner panel. It is important that the hemming results in a firm, vise-like grip of the flanges of the inner panel between the outer panel and its marginal flanges and that the shape and dimensions of the overall assembly are held within prescribed tolerances.
In the prior art, such hemming has often been accomplished in two separate stages often using two sets of dies mounted in two successive presses. The inner reinforcing panel is nested within the unitary outer panel which is fixtured on an anvil die on a base of a prehemming machine. Upon fixturing the assembly, a tool of the machine, commonly referred to as a hem or hemming blade (also referred to herein as a hem or hemming tool or die), engages and bends an edge of the outer panel to an acute included angle with respect to the outer panel. Prehemming is sometimes referred to as “fortyfive-ing” because the angle of the flange becomes about forty-five degrees with respect to the general plane of the outer panel. After prehemming all edges to be joined, both panels are released, transferred to, and fixtured in a second hemming machine where a second hemming blade completely bends the prehemmed edge of the outer panel over the peripheral edge of the reinforcing panel to secure and attach the panels together as a unitary structural member for assembly on a vehicle. This second stage is often referred to as full-hemming.
An obvious disadvantage of the two-stage, two-machine system is the need for two machines which require a significant amount of floor space as well as time and labor resources required for moving the assembly out of one press and into the second press. Because of these disadvantages, the two-stage, two-press system was improved upon and eventually evolved into a single station, two press system where prehemming and full-hemming occur without the need for re-fixturing the assembly between stages. Typically, a plurality of both prehemming and final hemming machines were respectively grouped around the periphery of a panel to perform all prehemming and full-hemming operations for one assembly either sequentially or substantially simultaneously.
After further improvements, hemming machines were designed to perform both the prehem and final hem operation in a single station, single machine system. Hemming machines of this type vary in the kind of mechanism used and the manner of carrying out the hemming operations.
One group includes machines having linkage driven hemming blades, machines having one rotary hemming blade driven by another linear driven hemming blade, and machines having one hemming blade telescopingly hem from within the prehemming hemming blade. Representative of this group are the following patents: U.S. Pat. No. 1,693,643 to D'Ardenne, U.S. Pat. No. 5,404,742 to Wilson et al, and U.S. Pat. No. 3,903,934 to Vizy.
Another group includes machines having hemming blades that traverse generally linearly in one or two directions. Hemming machines of this type, such as U.S. Pat. No. 3,143,095 to Tribe, may have a prehemming hemming blade traversing across the general plane of the outer skin and a fullhemming hemming blade traveling perpendicular to the plane of the outer skin. The obvious disadvantage of this type of machine is that it requires two hemming blades or dies, two directions of travel, extra cycle time for two operations, and a substantial amount of space around the assembly which prevents the hemming of internal edges. Alternatively, machines, such as U.S. Pat. No. 5,315,855 to Jackson, use a single hemming blade traversing in only the plane of the outer skin have been disclosed but still require a substantial amount of space preventing internal hemming and often result in a hem that is not firm, out of tolerance, and of low visual quality. Finally, there are machines, such as U.S. Pat. No. 1,961,582 to Eksergian, that travel only perpendicular to the general plane of the outer skin but still require substantial space around the assembly, two hemming blades, and do not create a quality hem.
Therefore, it is desirable to provide an improved apparatus and method for hemming sheet material together. The present invention contemplates such an invention that overcomes many of the problems of the prior art and others.